There are numerous instances where optically clear articles would be enhanced if the tendency of the articles to cause glare or to be obscured by the formation of a fog on a surface of the article could be reduced. For example, protective eyewear (goggles, face shields, helmets, etc.), ophthalmic lenses, architectural glazings, decorative glass frames, motor vehicle windows and windshields may all reflect light in a manner that causes an annoying and disruptive glare. Use of such articles may also be detrimentally affected by the formation of a moisture vapor fog on a surface of the article.
Glare is the undesirable reflection of light from a surface upon which the light is incident. In general, glare may be reduced by increasing the amount of light transmitted by the article, thereby reducing the amount of light which is available for reflection. Alternatively, the article surface can be modified (e.g., roughened, embossed, etc.) to cause the light to be reflected from the article more randomly and, therefore, with less glare.
Coatings which significantly increase the percent transmission of light and provide articles having very low reflection ("anti-reflective coatings") are known in the art. For example, U.S. Pat. No. 4,816,333 to Lange et al. (also assigned to 3M) discloses anti-reflective coatings of silica particles. The coating solution contains colloidal silica particles and optionally a surfactant ("Triton.TM. X-100" and "Tergitol TMN-6") to improve the wettability of the coating solution. U.S. Pat. No. 4,374,158 (Taniguchi et al.) discloses an anti-reflective coating using a gas phase treatment technique. The coating may optionally contain additives as surface controlling agents, such as silicone type surfactants. Various other types of anti-reflective coatings are disclosed in U.S. Pat. Nos. 2,366,516; 3,301,701; 3,833,368; 4,190,321, 4,271,210; 4,273,826; 4,346,131 and 4,409,285; by Cathro et al. in "Silica Low-Reflection Coatings for Collector Covers by a Dye-Coating Process," Solar Energy, Vol. 32, No. 5, pp. 573-579 (1984); and by J. D. Masso in "Evaluation of Scratch Resistant and Anti-reflective Coatings for Plastic Lenses," Proceedings of the 32nd Annual Technical Conference of the Society of Vacuum Coaters, Vol. 32, p. 237-240 (1989). None of these anti-reflective coatings produce a durable anti-fog coating.
In general, fog formation occurs under conditions of high humidity and high temperature or at interfacial boundaries where there is a large temperature and humidity difference. Coatings which reportedly reduce the tendency for surfaces to "fog up" (i.e., anti-fogging coatings) are known. For example, U.S. Pat. No. 3,212,909 to Leigh discloses the use of ammonium soap, such as alkyl ammonium carboxylates in admixture with a surface active agent which is a sulfated or sulfonated fatty material, to produce an anti-fogging composition. U.S. Pat. No. 3,075,228 to Elias discloses the use of salts of sulfated alkyl aryloxypolyalkoxy alcohol, as well as alkylbenzene sulfonates, to produce an anti-fogging article useful in cleaning, and imparting anti-fog properties to various surfaces. U.S. Pat. No. 3,819,522 to Zmoda, discloses the use of surfactant combinations comprising derivatives of decyne diol as well as surfactant mixtures which include ethoxylated alkyl sulfates in an anti-fogging window cleaner surfactant mixture.
Japanese Patent Kokai No. Hei 61994!-41335 discloses a clouding and drip preventive composition comprising colloidal alumina, colloidal silica and an anionic surfactant.
U.S. Pat. No. 4,478,909 (Taniguchi et al.) discloses a cured anti-fogging coating film which comprises polyvinyl alcohol, a finely divided silica, and an organic silicon compound, the carbon/silicon weight ratio apparently being important to the film's reported anti-fogging properties. Various surfactants, including fluorine-containing surfactants, may be used to improve the surface smoothness of the coating.
Other anti-fog coatings incorporating surfactants are described in U.S. Pat. Nos. 2,803,552; 3,022,178 and 3,897,356. "Anti-fog Antistat Eases Processing Problems," Modern Plastics, October 1988, discusses antistat agents, including alkyl sulfonates, and anti-fog agents for use in plastic films. Furthermore, American Cyanamid Industrial Chemical Division markets "Aerosol.TM. OT Surface Active Agent" (dioctylsodium-sulfosuccinate), which is advertised as useful to prepare an anti-fog composition for direct application to glass.
None of the above-described coatings which reduce the tendency for an article to fog have anti-reflective properties. Furthermore, in general, the anti-fog compositions of the prior art rely on high solution concentrations (e.g., in excess of 0.2 percent, and typically in concentrations in excess of 5 percent by weight) of surfactant and other organic additives to provide an anti-fog effect. When used at such high concentrations, the surfactants and other organic additives would interfere with and significantly reduce the anti-reflective properties provided by porous coatings, such as metal oxides.
Face masks and shields which are described as having anti-fog and anti-glare properties are known. For example, the "SHIELDMATE" by IREMA U.S.A. Ltd. of Chicopee, Mass. is described in U.S. Pat. No. 4,944,294 (Borek). The hospital face mask is described as including a transparent plastic eye shield coated with any suitable anti-fogging, anti-glare silicone agent, such as a dimethylsiloxane polymer.
World Patent Application No. 89/10106 (Russell) discloses a surgical mask/face shield combination. The face shield is coated with an anti-fog coating, such as that described in U.S. Pat. No. 4,467,073. These coatings are made by combining, for example, polyvinylpyrrolidone, a surfactant, and a curable isocyanate functional prepolymer. Additionally, Infection Control Products, Inc., markets the "AGAFAR.TM. Adjustable Flip-Up Face Shield" which is advertised as being anti-glare, anti-fog and anti-reflective. However, none of these products utilize a porous coating and none display an increase in transmission of visible light through the coated article of more than 2 to 3 percent greater than the uncoated article. It is understood that an increase in percent transmission corresponds to a decrease in percent reflection, provided the sample is transparent (i.e., non-light-absorbing and not hazy). Accordingly, a need exists for a coating composition which will impart anti-fog properties to a substrate coated therewith, while increasing the percent transmission, and correspondingly decreasing the percent reflection, of incident light through the substrate, such that the substrate is truly "anti-reflective."